Conventionally, there have been known image forming apparatuses configured to deflect a light beam emitted from a light source by a rotating polygonal mirror, and scan a photosensitive member by the light beam deflected by the rotating polygonal mirror, thereby forming an electrostatic latent image on the photosensitive member. Such image forming apparatuses include an optical sensor configured to detect the light beam deflected by the rotating polygonal mirror. The image forming apparatuses control the light source to emit the light beam therefrom based on a synchronization signal generated by the optical sensor, and match write start positions of electrostatic latent images (images) with each other in a direction in which the light beam scans the photosensitive member (a main scanning direction).
There are also known image forming apparatuses including a light source in which a plurality of light emitting elements configured to emit light beams is arranged as illustrated in FIG. 7A to increase an image forming speed and a resolution of an image. In FIG. 7A, an X-axis direction corresponds to the main scanning direction, and a Y-axis direction corresponds to a direction in which the photosensitive member rotates (a sub-scanning direction). In such image forming apparatuses, during an assembling process at a factory, the light source is rotated in a direction indicated by an arrow in FIG. 7A to adjust a distance between the light emitting elements in the Y-axis direction. By rotating the light source in this manner, a distance between exposure positions of the light beams emitted from the respective light emitting elements in the sub-scanning direction on the photosensitive member is adjusted to a distance corresponding to a resolution of the image forming apparatus.
The rotation of the light source in the direction indicated by the arrow illustrated in FIG. 7A changes both the distance between the light emitting elements in the Y-axis direction and the distance between the light emitting elements in the X-axis direction. Therefore, conventional image forming apparatuses cause each of the light emitting elements to emit a light beam at a timing determined for each of the light emitting elements based on the synchronization signal generated by the optical sensor to match the write start positions of electrostatic latent images with each other in the main scanning direction.
During the above-described assembling process, an angle by which the light source is rotated (an adjustment amount) varies for each image forming apparatus depending on how the light source is installed at the image forming apparatus and optical characteristics of optical members such as a lens and a mirror. Therefore, the distance between the light emitting elements in the X-axis direction after the rotational adjustment of the light source may not be the same among a plurality of image forming apparatuses. If a same timing is set for all image forming apparatuses as the light beam emission timing set for each light emitting element based on the synchronization signal generated by the optical sensor, this may result in generation of an image forming apparatus in which the write start positions of electrostatic latent images in the main scanning direction are out of alignment in the main scanning direction.
To prevent such misalignment among the write start positions of electrostatic latent images in the main scanning direction, which would be caused by rotating the light source during the assembling process, Japanese Patent Application Laid-Open No. 2008-89695 discusses an image forming apparatus that generates a plurality of horizontal synchronization signals by light beams respectively emitted from a first light emitting element and a second light emitting element, and sets a timing at which the second light emitting element emits a light beam relative to a timing at which the first light emitting element emits a light beam based on a difference between timings at which the plurality of horizontal synchronization signals is generated.
However, the image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2008-89695 has the following issue. During image formation, heat is generated at a motor that drives a rotating polygonal mirror, and the temperature of a lens disposed near the rotating polygonal mirror increases due to the influence of the heat. The increase in the temperature of the lens causes a change in the optical characteristics of the lens such as a refractive index of a light beam in the main scanning direction. The change in the optical characteristics of the lens causes a change in a relative positional relationship among image forming positions of a plurality of light beams on a photosensitive member, like a change from a state illustrated in FIG. 7B to a state illustrated in FIG. 7C (or from FIG. 7C to FIG. 7B). With the change in the optical characteristics and the change in the relative positional relationship among the image forming positions of the plurality of light beams on the photosensitive member during image formation in this manner, a misalignment occurs among write start positions of electrostatic latent images formed by the light beams emitted from the respective light emitting elements.